Triggering circuit for spark gap assemblies



June 30, 1970 J. s. KRESGE ETAL 3,518,492

TRIGGERING CIRCUIT FOR SPARK GAP ASSEMBLIES Filed May 15, 1968 2Sheets-Sheet L3 m kg 13 5 g [inf/2 0/12 06 Jams 'fires e T aM/ mfi/z:VHLVE lea/57oz -15 20a 5 w 2 7 ,4

United States Patent 01 3,518,492 ice Patented June 30, 1970 3,518,492TRIGGERING CIRCUIT FOR SPARK GAP ASSEMBLIES James S. Kresge and StanleyA. Miske, Jr., Pittsfield,

Mass., assignors to General Electric Company, a corporation of New YorkFiled May 13, 1968, Ser. No. 728,604 Int. Cl. H02h 9/06 US. Cl. 317-6831 Claims ABSTRACT OF THE DISCLOSURE A plurality of series connectedmain spark gaps are connected in shunt circuit relation with a voltagegrading impedance network which includes a trigger gap that is connectedthough frequency responsive coupling means in shunt relation withapproximately one-half of the series connected main spark gaps. Thetrigger gap is operative, as a substantially linear function of avoltage impressed across the series connected gaps, to cause the maingaps to spark over in cascade fashion. The triggered control networkallows use of a large number of main gaps but with a controlled totalsparkover very much less than the sum of the sparkover of the individualmain gaps, thus, providing a desirably high ratio of reseal voltage tosparkover voltage for the main gap series circuit.

This invention relates to lightning arrester spark gaps and moreparticularly to improvements in triggered sparkover control circuitryfor effecting cascaded sparkover of a plurality of series connected mainspark gaps in a current limiting lightning arrester.

Conventional lightning arresters generally comprise a non-linearvalve-type resistance element, or elements, electricall connected inseries with a spark gap assembly between a pair of suitable terminalsmounted on opposite ends of an insulated arrester housing. In operation,when such arresters are electrically connected to protect a power systemfrom damage due to overvoltage surges, the series circuit in thearrester is connected between the system and ground, to alford adischarge path to ground for the surge current. Basically, the arrestersvalve type resistance elements present a high impedance to normal linevoltage but a much lower impedance to high voltage surges. The seriesspark gaps, in their non-conducting condition, serve to seal the systemfrom ground so that under normal operating conditions negligible currentis conducted from the protected system through the lightning arrester toground. When the system protected by the arrester, is subjected to ahigh voltage surge that is then impressed across the terminals of thearrester, the spark gaps are sparked over and the valve type resistancepresents a very low impedance to this surge, thus, discharging the surgeto ground through the arrester. Following the discharge of the surgecurrent, the arrester is normally resealed by the combined action of thevalve resistor increasing its resistance to the lower voltagepower-follow current, and the arc stretching action of horn gapsadjacent the individual main spark gaps.

Until recent times, conventional arresters for AC. systems allowedconduction of power-follow current until the next natural current zerofollowing an overvoltage surge discharge. However, in modern arrestersemploying so-called current limiting gaps a back voltage is generated inthe gaps which aids in reducing the current through the arrester suchthat a current zero and consequent arrester clearing is accomplishedbefore a natural current zero. In either case, the clearing, or resealcapability of an arrester, is related to the number of series gaps itcontains. That is, the greater the number of series gaps the higher willbe the reseal capability.

The advent of the current limiting gap has made it possible to developarresters capable of operating on direct current (D.C.) systems becausesuch gaps are capable of forcing a current zero even though a naturalcurrent zero does not exist. However, because of the fact that a naturalcurrent zero does not exist on a D.C. system, it is particularlynecessary to provide a consistent and reliable reseal capability in D.C.arresters.

To attain continued improvements in arrester protective levels, therehas been considerable incentive to improve, or increase, the ratio ofreseal to sparkover capability of the series gaps in arresters. Onebasic way of doing this is to increase the number of series gaps whilestill retaining a reasonable voltage sparkover level. In the prior art,suitably high ratios of reseal voltage to sparkover voltage have beenattained for alternating current lightning arresters by utilizingvarious combinations of linear and nonlinear resistances in spark gapassembly voltage grading circuits of the arresters to cause cascadedsparkover of the individual gaps in the assembly, thereby lowering theassemblys sparkover voltage to a level near the spark gaps inherentreseal voltage. It is also known to provide a trigger gap to furtherlower the sparkover voltage of such linear and nonlinear resistancegraded spark gap assemblies. An example of 81161143. triggered spark gapassemblies. An example of such a triggered spark gap control arrangementis disclosed in copending US. patent application Ser. No. 681,991,Carpenter, filed December 1, 1967 and assigned to the assignee of theinvention described herein.

Even with the control arrangements of the prior art, it is difficult toconsistently attain reliable reseal of lightning arresters at apredetermined voltage level. This problem has been partially overcome bycarefully grading the voltage distribution across the individual gaps ofarrester spark gap assemblies, but this is an expensive procedure andwhen it is used it is difficult to predict with accuracy the resealcharacteristics that will be produced for individual arresters. Ofcourse, it is desirable to be able to consistently predict these resealcharacteristics, as well as the ratio of reseal to sparkover volt-age,for arresters of any given rating, so that they can be used withassurance to protect power systems having operating voltages andprotective insulation characteristics. However, as noted above, with thesparkover control techniques of the prior art, the problems ofconsistent and predictable reseal at a suitably high ratio to sparkovervoltage are substantially solved for lightning arrester applicationswith alternating current circuits, because the periodic zero voltagelevels presented by the power-follow current from such circuits allowsthe arresters to be rescaled against a relatively high line voltage,even though the arresters could not be resealed with these techniquesagainst such a line voltage if it were a direct current voltage. A majoradvantage of our invention is that it provides means for regulating thesparkover and reseal voltages of a current limiting lightning arresterto afford a suitably high ratio of reseal to sparkover voltage for thearrester when used to protect a system transmitting either alternatingor direct current.

Briefly stated, in one form of our invention, series connected mainspark gaps of a lightning arrester spark gap assembly are shunted by animpedance, which includes a trigger gap that is shunt connected by afrequency responsive coupling circuit, across approximately half of themain spark gaps in the spark gap assembly. The impedance network causesthe trigger gap to spark over at about half the voltage level requiredto sparkover the untriggered main spark gaps. Sparkover of the triggergap initiates the cascaded sparkover of the main spark gaps so that avoltage surge is discharged through the arrester to ground. A preionizermeans is provided to improve the consistency ofsparkover of the triggergap. The trigger gap is rescaled or extinguished when the main gaps aresparked over and because of the short duration and low magnitude ofcurrent which it has caused it deionizes very quickly and will not-bereignitcd by the voltage developed across the main gaps as they reseal.

An object of the invention is to provide a lightning arrester having animproved ratio of reseal voltage to sparkover voltage.

' Another object of the invention is to provide a triggered voltagegrading network for a spark gap assembly wherein the voltage across thetrigger gap is a linear function of the voltage across the spark gapassembly.

A further object of the invention is to provide a plurality of seriesconnected spark gaps with a triggered, voltage' grading impedancenetwork, which is operative to cause the cascaded sparkover of theseries connected spark gaps while affording uniform voltage distributionacross the individual spark gaps.

Still another object of the invention is to provide a multigap spark gapassembly having a consistent sparkover voltage level, which is notaffected by minor variations in the respective spacings or dimensions ofthe individual 'main spark gaps in the assembly.

The invention will be better understood, and additional 'objects andadvantages inherent in it will be more fully appreciated, from thefollowing detailed description taken .in conjunction with theaccompanying drawings, and the scope of the invention will be pointedout with particularity in the appended claims.

In the drawings:

FIG. 1 is a circuit diagram and schematic illustration of a preferredembodiment of the invention shown with respect to a power transmissionsystem.

FIG. 2 is a side elevation of a preferred embodiment of the invention.

FIG. 3 is a fragmentary circuit diagram of an alternative embodiment ofthe invention.

Referring now to FIG. 1 of the drawing, there is shown a conductor 1 ofa high voltage power transmission system connected by a second conductor2 to a terminal 3. It will be understood that the terminal 3 and asecond terminal 4 diagrammatically represent respective terminalsmounted on oposite ends of a suitable lightning arrester housing (notshown). Within the arrester housing there is disposed a valve typenonlinear resistor, schematically shown by the block 5, series connectedby a suitable conductor 6 to a plurality of spark gap assemblies, whichare illustrated respectively by the dotted outlines A and B. Thecomponent parts in the respective spark gap assemblies A and B aresubstantially identical in structure and function; accordingly, likereference numerals will be used to designate similar parts in theassemblies A and B. It will also be understood that although only twospark gap assemblies are shown in the drawing for purposes ofsimplifying the description of the invention, any suitable number ofsuch assemblies may be employed in a series connected arrangement toform lightning arrester structures having various given ratings, as iswell known in the lightning arrester art.

Each of the spark gap assemblies A and B contains a plurality of seriesconnected main spark gaps 7, 8, 9, and 10 and one (or more)coil-shunting spark gap 11. Of course, alternating-protective means,such as a nonlinear valve type resistor, could be used instead of thespark gap 11, and suchalternatives are within the scope of theinvention. Electrically connected in series with the main spark gaps7-10 and in shunt relation with the coil gap 11 is a suitableelectromagnetic coil 12 that serves to develop an electromagnetic fieldsubstantially perpendicular to a plane through the main spark gaps 7-10,and the horn gaps associated with these spark gaps, when powerfollowcurrent flows through the arrester from the transmission line 1 toground. The magnetic fields thus developed by the respective coils l2electrodynamically reinforce the arc-moving action of the respectivehorn gaps that comprise an integral structural part of the main sparkgaps 7-10.

In order to provide uniform distribution of the voltage impressed acrossthe spark gap assemblies A and B in their nonconducting condition, whilesimultaneously controlling the cascaded discharge of the respective mainspark gaps 7-10, each spark gap assembly, A and B, is provided with animpedance network including the following components: A plurality ofmain spark gap ionizer means, which may be blocksof suitable preionizingmaterial such as mica, or capacitances 13, which are shunt connectedrespectively across each of the main spark gaps 7-10 and electricallyconnected in series with each other. (The capacitances 13 are shown inFIG. 1 as: 13a, 13b, 13c and 13a, in assembly A, and as 13a, 13b 13c and13d, in assembly B so that representative values individual to therespective units may be referred to hereinafter; however, in the generaldescription of the invention these capacitances will simply be referredto with the identifying number 13.) A second series circuit including apair of nonlinear resistors 14 and a pair of linear resistors 15electrically connected in series with a capacitor 16. And, conductors 17which serve to cross connect the respective elements 14, 15, and 16 inshunt relation with the capacitance ionizers 13 and the electromagneticcoil 12, as shown in FIG. 1. In addition, the two uppermost main sparkgaps 7 and 8 of assembly A and the two lowermost main spark gaps 9 and10 of assembly B are shunted respectively by capacitors 18 and 19.

Pursuant to our invention, a trigger gap 20 is electrically connected asshown in FIG. 1 through a frequency responsive coupling means 21,comprising a paralleled capacitor 21a and resistor 21b, to a terminal 22that forms a common junction point between spark gap assemblies A and B.Trigger gap 20 may also be energized through a voltage grading circuitcomprising a pair of substantially identical linear resistances 23, 23electrically connected in series with a pair of substantially identicalnonlinear resistors 24, 24 which are connected between the outermostterminals of spark gap assemblies A and B, as shown in FIG. 1.

The triggered control circuit, as described thus far, can be operated toprovide the basic objectives of our invention; however, to assureconsistent and accurately predictable spark-over of the trigger sparkgap 20, in the preferred form of the invention, a preionizer spark gap25 is disposed adjacent trigger gap 20 to ionize the trigger gap 20 at avoltage lower than the un-ionized sparkover voltage of the trigger gap,when a predetermined voltage is impressed across the lightning arresterterminals 3 and 4. Specifically, a nonlinear resistor 26 is electricallyconnected in series with a linear resistor 27 and these two componentsare connected in series with the preionizer gap 25, which is shuntconnected across a linear resistor 28 to thus form a third seriescircuit that is in shunt relation with the two outermost terminals ofspark gap assemblies A and B. In order to provide the desired and propersurge resonse of the preionizer gap 25 either the combination ofresistors 26 and 27, or either one of these resistors separately, may beparalleled by a capacitor 29. In the preferred form of the inventionillustrated in FIG. 1, only the resistor 27 is shunted by the capacitor29'.

Although the triggering arrangement just described is a preferred formof the invention, it will be understood that alternative means may beemployed to preionize trigger gap 20. For example, as shown in FIG. 3 acorona type ionizer gap 20a is shunted by a resistor 20!; and disposedin a series circuit with coupling capacitor 20c, adjacent to and shuntedacross trigger gap 20. In this embodiment of the invention, since ahigher sparkover voltage is desired, the spacing of gap 20 is largerthan the spacing of the gap 20 shown in FIG. 1. Also, components 23through 29 (shown in FIG. 1) are not necessary, and are omitted fromthis simplified embodiment of the invention. Of course, this modifiedform of preionized trigger circuit may be connected to like numberedterminals 22 and 31, of the spark gap assembly arrangement shown in FIG.1 as assemblies A and B, as indicated. In operation, the

' triggering sequence of this alternative embodiment of the invention isidentical to that obtained with the triggering and ionizing circuitshown in FIG. 1.

It should be noted that the respective components for the frequencyresponsive coupling means 21 should always be selected to provide a timeconstant that will allow capacitor 21a to be discharged through resistor21b in the shortest time interval anticipated between successiveovervoltage surges on the particular protected transmission line 1, withwhich the invention is to be used. In addition, it will be appreciatedthat the value of linear resistor 27 should be sufficient to limit thecurrent through preionizer gap 25 both when a surge voltage is passed atrelatively low impedance through nonlinear resistor 26, and when reversevoltage builds up across spark gap assemblies A and B as they reseal, sothat the gap 25 will not be eroded, causing it to change its sparkovervoltage level. It will also be appreciated that in the preferredembodiment of our invention shown in FIG. 1 the sparkover voltage oftrigger gap 20 is preset to approximately one-quarter of the untriggeredsparkover voltage of the series connected spark gap assemblies A and B,since in this embodiment the trigger gap 20 is shunt connected acrossapproximately one-half of this series circuit, i.e., across spark gapassembly B. Of course, as will become apparent from the followingdescription of the operation of our invention, in other embodiments ofthe invention where trigger gap 20 is electrically connected in shuntrelation with a diiferent proportion of the main series circuitcomprising the series connected main spark gap assemblies of thelightning arrester, the trigger gap would be set to sparkover at adifferent proportion of the over-all untriggered sparkover voltage ofthe series connected spark gap assemblies. Also, pursuant to theteaching of our invention, the preionizer gap 25 is adapted to sparkoverand ionize trigger gap 20 when approximately 25 percent of the sparkovervoltage of trigger gap 20 is developed by linear resistor 28 across theionizer gap 25. Of course, this ratio of sparkover between the ionizergap 25 and trigger gap 20' can be varied within wide limits in diiferentembodiments of the invention. Finally, for the remainder of thediscussion of the invention herein, it will be assumed that the mainspark gap assemblies A and B are of the current limiting type althoughit will be evident to those well versed in the art that the basicprinciples of this invention could also be applied to any type of seriesconnected gaps. The current limiting type of gap is stressed herebecause that type gap is directly applicable for use in DC. arresterswhich is one of the more advantageous applications of the invention.

An understanding of the operation of the preferred embodiment of ourinvention will be facilitated by a description of the sequence of eventsthat occur in the circuit shown in FIG. 1 when a high voltage surge isdischarged to ground followed by the subsequent rescaling of the sparkgap assemblies A and B. Assuming a normal line voltage is present ontransmission line 1, the spark gaps 7-10 as well as the trigger gap 20and ionizer gap 25, are all in their unsparkedover condition, i.e., theyare not conducting. Accordingly, since the nonlinear resistors 14, 24and 26, respectively disposed in the three shunt connectetd seriescircuits between the terminals 3 and 4 of the protective lightningarrester, present a high impedance to line voltage, no appreciablecurrent is discharged from the transmission line 1 through the arresteror its impedance network to the ground terminal 4. In this condition, itshould be understood that the impedance network components 12-19 areselected to grade the voltage across the spark gaps 7-11 so that thevoltage across the endmost gaps 7 and 10 of each spark gap assembly Aand B is slightly greater than the voltage across the inner main gaps 8and 9. The grading is so chosen to give a slight upset in voltagedistribution at normal operating voltages because it is desirable tohave a uniform distribution of voltage occur between the gaps 710 at theclearing voltage of the arrester, which voltage is somewhat greater thanthe normal operating voltage. At the same time, the voltage acrosstrigger gap 20 is always maintained equal to one-half of the normal linevoltage since the pairs of resistors 23 and 24 are identical in value,respectively. A very small part of the normal line voltage appearsacross the ionizer gap 25, since linear resistor 28 comprises a smallpart of the series impedance in the circuit including nonlinear resistor26 and linear resistor 27.

Now, assuming an overvoltage surge is transmitted to terminal 3 fromtransmission line 1, the voltage across ionizer gap 25 risesdisproportionately fast in relation to the voltage rise across triggergap 20 and the main spark gaps 7-9, because the grading circuit for gap25 is unbalanced. Specifically, it will be noted that the impedance ofnonlinear resistor 26 declines rapidly at the higher voltages while theimpedance of linear resistor 28 remains constant; therefore, a largerpercentage of the over-all voltage is dropped across resistor 28 and theionizer gap 25 when a high voltage surge occurs. Accordingly, when avoltage equal to about 25 percent of normal line voltage is developedacross the ionizer gap 25 it sparks over and, thus, preionizes triggergap 20. If the overvoltage surge continues to rise to high enough level,the voltage across trigger gap 20 also continues to rise and when thevoltage grading network comprising the pairs of resistors, 23 and 24,causes a potential approximately equal to the normal line voltage to beapplied across the trigger gap 20, this gap sparks over. Since capacitor21a in the frequency responsive coupling means 21 cannot support aninstantaneous voltage, the sparkover of trigger gap 20 essentiallyconnects the terminal 22, between spark gap assemblies A and B, toground potential and the entire surge voltage, i.e., approximately twicenormal line voltage is suddenly impressed across spark gap assembly A.The main spark gaps in spark gap assembly A are then sparked over by thegrading network 12-19 in the sequence; spark gap 9, spark gap 10, sparkgap 8, spark gap 7 and finally the coil gap 11. The complete sparkoverof the main spark gaps 7-10 in spark gap assembly A causes capacitor 21ato be fully charged to the voltage then present at terminal 3, or toapproximately twice the normal line voltage. This overvoltage issufficient to cause the main spark gaps 7-10 in spark gap assembly B tosparkover sequentially in the same order as the main spark gaps inassembly A sparked over. The overvoltage surge is then discharged toground through valve resistor 5 which presents a low impedance to thehigh voltage.

As soon as spark assembly B sparks over completely, it short circuitsthe trigger gap 20 and, thus, rapidly clears the arc across this gap 20.Following the discharge of the overvoltage surge to ground, power-followcurrent from transmission line 1 is momentarily discharged through thesparked over main gaps 7-10, but due to the current limiting action ofthe sharply increased impedance value presented by nonlinear resistor 5to these much lower voltages, and the arc stretching, current limitingeffect of the horn gaps associated with each of the main spark gaps7-10, a reverse voltage is rapidly built up across the spark gapassemblies A and B. Because of the preset sealing characteristics of themain spark gaps 7-9, when this reverse voltage reaches approximatelypercent of the triggered sparkover voltage of spark gap assemblies A andB, i.e. approximately of normal line voltage, the main spark gaps 7-10clear and the lightning arrester is resealed. The impedance network,13-15 and 17-18 uniformly distributes the line voltage across the mainspark gaps 7-10 so that none of these gaps can restrike. Trigger gapdoes not restrike as the reverse voltage builds up above normal linevoltage to the point where it reseals spark gap assemblies A and B,because it is designed to spark over only when twice normal linevoltage, rather than 80 percent of that value is present at terminal 3.On the other hand, the ionizer gap may be sparked over by the reversevoltage as it exceeds normal line voltage, but the current limitingaction of linear resistor 27 prevents damage to the electrodes ofionizer gap 25 during the short interval that this gap is sparked overpiror to the time that line Voltage stabilizes at its normal valuefollowing reseal of main gaps 7-9.

It should be appreciated from the foregoing description of the circuitryand operation of our invention, that the voltage across trigger gap 20is a linear function of the voltage across the series circuit includingspark gap assemblies A and B. Thus, the bias effect normally presentwhen linear-nonlinear triggered control circuits are utilized to cascadespark gap assemblies of lightning arresters is minimized with ourinvention. In other words, the

sparkover voltage of trigger gap 20 is essentially independent ofwhether the voltage is increased from zero to its sparkover value, orfrom a previously set DC. bias value of either the same or oppositepolarity with respect to the overvoltage surge applied to the triggergap 20.

A significant advantage of our invention resides in the fact that theoperating components required to practice the invention are small enoughto enable them to be placed in modular form on a spark gap assemblywithin a lightning arrester housing. An appreciation of this fact willbe attained by reference to FIG. 2 wherein there is illustrated a sparkgap assembly A and B embodying the operating components discussed abovewith reference to FIG. 1. Like reference numerals are used to designatelike parts in FIG. 2, with relation to those shown in FIG. 1. Thus, thespark gap assembly A and B comprises a pair of end terminal plates and31 mounted on opposite ends of a plurality of stacked porous insulatingdiscs 32 through 43. The main spark gaps 7-10 are disposed respectivelybetween adjacent pairs of insulating discs 32-43 in spark gap arcingchambers defined by the abutting upper and lower surfaces of therespective discs, in any suitable manner. For example, the discs 32-43may be assembled in the manner described more fully in U.S. Pat. No.3,131,273, E. W. Stetson, issued Sept. 29, 1964 and assigned to theassignee of the present invention. Although the main spark gaps 7-10 arenot visible in FIG. 2, their respective locations are designated by thereference numerals 7, 8, 9 and 10, and the gaps are electricallyconnected in series, with electromagnetic coils 12, as shown by thecircuit diagram in FIG. 1. Bus bars 44 and 45 are connected respectivelyat 44' and 45' to end terminal plates 30 and 31 to form an electricalconnection between these respective members. The bus bars 44 and 45 aremounted on opposite ends of an insulating board 46 which shields theoperating components of the circuit of our invention from the housing ofspark gap assemblies A and B. Electrically connected in a series circuitbetween the bus bars 44 and 45 are a pair of nonlinear resistors 24 anda pair of identical linear resistors 23. The parallel coupling circuit21, comprising capacitor 21a and linear resistance 21b, is electricallyconnected to terminal 22 and the upper end of spark gap electrode 20,comprising electrodes 20' and 20". Shunting the voltage gradingimpendance network for trigger gap 20 is another series circuitcomprising a nonlinear resistor 26 electrically connected in series witha linear resistor 27 and a second linear resistor 28 between bus bars 44and 45. Shunted across linear resistance 28 is ionizer gap 25, which isdisposed adjacent the trigger gap 20. Electrically connected in serieswith the ionizer gap 25 and in shunt with the resistor 27 is capacitor29. As noted above, a plurality of such spark gap assemblies may be usedin any given lightning arrester to determine the rating of the arresteras is well known in the lightning arrester field. The important featureto be noted here is that our invention affords an ideal modulararrangement that is readily adaptable for use with the conventionalmodular concept utilized in present day lightning arresters.

It will be understood that specific values of resistance and capacitancefor the respective elements shown in the circuit diagram of FIG. 1 andthe illustration of FIG. 2 will be determined by the design parametersof a particular lightning arrester, and the operating voltage of thesystem with which the invention is to be utilized. However, tofacilitate an understanding of the invention, representative values forthese various components are given below, by Way of example, for aparticular arbitrarily selected operating rating and a ratio of resealto sparkover voltage of approximately percent, and further assuming thattwo spark gap assemblies A and B are to be used, each arbitrarily ratedfor clearing approximately six kilovolts DC. We have found that toconsistenly provide such clearing the gap spacings of main spark gaps7-10 should be approximately .044 inch. With such main spark gapspacing, by selecting the values of nonlinear resistors 14 and linearresistors 15 to effect a uniform distribution of voltage across the mainspark gap at the desired clearing voltage of 6 kilovolts each forassemblies A and B, and using values of capacitance as shown in thefollowing table, it is possible to obtain a sparkover level for theseries connected main spark gap assemblies A and B of approximately 14kilovolts each for relatively slow frequency high voltage surges, and asparkover level of 11-12 kilovolts for relatively high frequency voltagesurges. To obtain these sparkover values, the following capacitances canbe utilized:

Capacitors: Rating in picofarads 13a 12 With this arrangement, since itis desired to have a ratio of reseal to sparkover voltage ofapproximately 80 percent, the sparkover voltage for the series connectedassemblies A and B should be approximately 16 kilovolts. Accordingly,the trigger gap 20 is manually preset to sparkover at 8 kilovolts. Asnoted above, the voltage grading network comprising the linear resistors23 and nonlinear resistors 24 serves to equally divide the voltageapplied across the spark gap assemblies A and B so that one-half of thesupplied voltage is impressed across the trigger gap 20. Therefore, thevalues of the resistors 23 and 24 can vary between fairly wide limits,the important parameter being that they divide the voltage equally. Itis also desirable to maintain the value of linear resistors 23 largeenough to limit the sparkover current so that the contacts of triggergap 20 will not be eroded enough to cause them to change their sparkoverlevel. The values of resistance in the ionizer circuit including ionizergap 25 also may vary within wide limits, it only being necessary toselect the relative values of resistors 27 and 28 so that when a highfrequency, high voltage surge is impressed across this series circuitthe ionizer gap 25 will spark over when the voltage level across thespark gap assemblies A and B is approximately 13 kilovolts, or roughly75 percent of the voltage level required to sparkover the trgiger gap20. Again, the magnitude of linear resistor 27 should be suificient toprevent erosion of the ionizer gap 25 when it is sparked over. Thisfeature is particularly important in the ionizer circuit because, asnoted above, the ionizer gap remains sparked over for a relatively longperiod of time, both while the high voltage surge is being dischargedand during the reverse voltage build up that reseals the sparkassemblies A and B.

The following summary is provided in order to facilitate a fullerunderstanding of the invention, as well as to demonstrate the presentnecessity of providing some of the solutions it affords, so thatlightning arrester technology can overcome existing barriers andcontinue its evolution. At the outset it was noted that lightningarresters must be manufactured to a given desired ratio of reseal tosparkover voltage, by way of example, a ratio of 80% was assumed indescribing preferred embodiment of the invention. It was then observedthat generally four series connected spark gaps, each being spaced toform a gap of .044 inch can, when uniformly graded, reliably clear orreseal against only about 6 kilovolts D.C. Therefore, to attain thedesired 80% ratio of reseal to sparkover voltage some means must beprovided to cause the series connected gaps to have an equivalentspankover of about 8 kilovolts total, whereas the sum of theirindividual sparkovers is about 20 kilovolts.

It was found that by using conventional linear-nonlinear resistancegrading techniques the sparkover of the four series-connected, .044 inchspaced gaps could only be reduced toapproximately 12 to 14 kilovoltswhile maintaining uniform voltage grading at a clearing capability of 6kilovolts. However, by employing the teachings of the present invention,two sets of such series connected gaps, which are in turn connected inseries, are controlled by means of a trigger gap to give a totalsparkover voltage of 16- kilovolts (or the desired 8 kilovolts perfour-gap set). Thus, a total of eight series connected gaps having atotal uniformly graded sparkover of about 40 kilovolts is controlled toyield a total sparkover voltage of 16 kilovolts with a circuit thatmaintains uniform voltage distribution, to enhance clearing or resealingof the arrester at a total voltage of 12 kilovolts.

While a particular embodiment of our invention has been shown anddescribed, it will be obvious to those skilled in the art that changesand modifications may be made therein without departing from theinvention and, therefore, it is intended by the appended claims to coverall such changes and modifications as fall within the true spirit andscope of the invention.

What -we claim as new and desire to secure by Letters Patent of theUnited States is:

1. A spark gap assembly comprising means defining a plurality of mainspark gaps electrically connected in a first series circuit, a triggergap connected in shunt relation with a first portion of said firstseries circuit including a predetermined number of said main spark gaps,impedance means electrically connected in series with said trigger gapin said shunt circuit relation, whereby a predetermined proportionalincrement of a voltage across said first series circuit is impressedacross said trigger gap, said trigger gap having a sparkover voltagesubstantially higher than the sparkover voltage of one of said main gapsand substantially lower than the sparkover voltage of the first seriescircuit when it is not triggered.

2. A spark gap assembly as defined in claim 1 wherein said predeterminednumber of main gaps comprises at least two of said gaps.

3. A spark gap assembly as defined in claim 1 wherein 10 saidpredetermined number of main gaps is approximately one-half of the totalnumber of gaps in said plurality of main spark gaps.

4. A spark gap assembly as defined in claim 3 wherein the sparkovervoltage of said trigger gap is equal to approximately one-quarter theuntriggered sparkover voltage of said first series circuit.

5. A spark gap assembly as defined in claim 2 including means forgrading a voltage across said first series circuit to distributesubstantially equal voltages across each of said main gaps at theirdesired reseal voltage rating.

6. A spark gap assembly as defined in claim 1 including preionizingmeans to preionize said trigger gap, and means to energize saidpreionizing means in response to a voltage across said first seriescircuit such that said trigger gap is ionized at a voltage lower thanits sparkover voltage.

7. A spark gap assembly as defined in claim 6 wherein said preionizingmeans comprises a preionizer spark gap disposed adjacent the trigger gapand adapted to sparkover at a voltage lower than the sparkover voltageof said trigger gap, and means electrically connecting said preionizergap in shunt relation with said first series circuit.

8. A spark gap assembly as defined in claim 2, in combination, with aninsulating lightning arrester housing, a nonlinear resistance materialdisposed in said housing, means for mounting said spark gap assembly insaid housing, electric terminals disposed adjacent opposite ends of saidhousing, and means electrically connecting said first series circuit ofsaid spark gap assembly in series with said nonlinear resistancematerial between said terminals.

9. The invention defined in claim *8 including electromagnetic means fordriving arcs formed between said main spark gaps toward the periphery ofthe spark gap arcing chambers in said housing.

10. The invention defined in claim 9 wherein said electromagnetic meanscomprises at least one conducting coil disposed around the spark gapassembly and electrically connected in series with said first seriescircuit through said assembly, and including protective means shuntingsaid coil to prevent it from being damaged by an overvoltage surge.

11. A spark gap assembly as defined in claim 1 wherein said impedancemeans is voltage responsive and adapted to offer a relatively lowimpedance to high voltages and to offer a relatively high impedance tolow voltages.

12. A spark gap assembly as defined in claim 5 wherein said means forgrading voltage comprises an impedance network including a plurality ofseries connected ionizer capacitances, each of said series connectedionizer capacitances respectively being shunt connected across one ofsaid main spark gaps.

13. In a lightning arrester, a plurality of spark gap assemblies, eachof said assemblies comprising a plurality of main spark gapselectrically connected in a first series circuit, means electricallyconnecting each of said first series circuits in series to form a largerseries circuit, at least one trigger gap connected in shunt circuitrelation with at least one portion of said larger series circuit, saidtrigger gap having a sparkover voltage substantially higher than thesparkover voltage of one of said main gaps and substantially lower thanthe sparkover voltage of said larger series circuit when it is nottrigered, a block of nonlinear resistance material, a pair of terminalsdisposed respectively adjacent opposite ends of said arrester, meanselectrically connecting said larger series circuit in series with saidblock of nonlinear resistance material between said terminals, and meansforming a voltage grading circuit between said terminals whereby apredetermined portion of a voltage impressed across said terminals isapplied across said trigger gap when at least some of said main sparkgaps are not sparked over.

14. The invention defined in claim 13 wherein the portion of said largerseries circuit shunted by the trigger gap comprises one of said firstseries circuits.

15. The invention as defined in claim 14 wherein the number of saidplurality of spark gap assemblies comprises two of said assemblies.

16. The invention as defined in claim '13 wherein said plurality ofspark gap assemblies comprises an even number of such assemblies, andwherein the portion of said larger series circuit shunted by the triggergap includes one-half of said assemblies.

17. The invention defined in claim 13 with preionizer means electricallyconnected between said pair of terminals and disposed adjacent saidtrigger gap, said preionizer means being operative to ionize saidtrigger gap at a voltage lower than the sparkover voltage of saidtrigger gap in response to a predetermined voltage being impressedacross said pair of terminals.

18. The invention defined in claim 13 with a plurality of impedancenetworks for controlling the voltage distribution of the main spark gapsin each of said assemblies when they are not sparked over, each of saidimpedance networks being electrically connected to form a second seriescircuit shunting said first series circuit in each of said respectiveassemblies.

19. The invention defined in claim 18 with a frequency responsivecoupling circuit electrically connected in series With the Shunt circuitincluding said trigger gap.

20. The invention defined in claim 18 with capacitive grading means ineach of said assemblies for further controlling the voltage distributionof the main spark gaps in said assemblies, each of said capacitivegrading means including a plurality of electrical capacitances ofvarious predetermined size connected to form a third series circuitthrough its respective spark gap assembly hunting the second seriescircuit in that assembly.

21. The invention as defined in claim 20 wherein each capacitance insaid plurality of electrical capacitances comprises a preionizer meanselectrically connected and mechanically positioned such that each of themain spark gaps is shunted respectively by a different preionizer meansdisposed in ionizing relation thereto.

22. The invention as defined in claim 21 with a frequency responsivecoupling circuit electrically connected in series with said trigger gapin the circuit establishng its shunt relation with said predeterminedportion of the larger series circuit, and including preionizer meanselectrically connected between said pair of terminals and disposedadjacent said trigger gap, said preionizer means being operative toionize said trigger gap at a voltage lower than its sparkover voltage inresponse to a predetermined voltage being impressed across said pair ofterminals.

23. The invention defined in claim 13 wherein the predetermined portionof voltage impressed across said spark gap is a linear function of thevoltage between said terminals when at least some of said main sparkgaps are not sparked over.

24. The invention defined in claim 23 wherein the means forming avoltage grading circuit for impressing a voltage across the trigger gapcomprises a second series circuit between said terminals shunting saidlarger series circuit and including series connected linear andnonlinear impedances, said trigger gap being shunt connected across apredetermined number of said impedances.

25. A lightning arrester comprising at least one nonlinear resistanceelement electrically connected in series with a plurality of seriesconnected spark gap assemblies, a predetermined number of saidassemblies comprising: a plurality of main spark gaps electricallyconnected in a first series circuit, at least one trigger gap connectedin shunt circuit relation with a predetermined number of said main sparkgaps, said trigger gap having a sparkover voltage substantially higherthan the sparkover voltage of one of said main gaps and substantiallylower than the sparkover voltage of said first series circuit when it isnot triggered, and means for energizing said trigger gap in response toa predetermined voltage being impressed across said lightning arrester.

26. A spark gap assembly as defined in claim 25 in combination withmodular mounting means in each of said assemblies for mounting saidtrigger gap and means for energizing said trigger gap in a predeterminedrelatively fixed relation with respect to said first series circuit.

27. A lightning arrester comprising a plurality of spark gap assemblieselectrically connected to form a discharge circuit, a predeterminednumber of said assemblies being provided with control means foraccurately regulating the sparkover voltage of the assembly, whereby thesparkover voltage of said discharge circuit is accurately regulated.

28. A lightning arrester as defined in claim 27 wherein said controlmeans comprises a trigger gap and means for energizing said trigger gapin response to a predetermined voltage being impressed across saidarrester, said trigger gap being adapted when energized to causecascaded sparkover of its assembly.

29. A lightning arrester as defined in claim 27 wherein the unregulatedsparkover voltage of each of said assemblies may vary between widetolerances, and wherein the regulated sparkover voltage of each of saidassemblies is maintained within substantially narrower tolerances bysaid control means.

30. A lightning arrester as defined in claim 27 wherein a predeterminednumber of said assemblies include a nonlinear valve resistance elementelectrically connected in series with said discharge circuit.

31. A lightning arrester as defined in claim 27 in combination with atleast one nonlinear resistance element electrically connected in serieswith said discharge circuit.

References Cited UNITED STATES PATENTS 2,890,389 6/1959 Carpenter et al.317-61 X 3,348,100 10/1967 Kresge 317 3,414,759 12/1968 Connell et al.317--70 X JAMES D. TRAMMELL, Primary Examiner US. or X.R. 31536; 317-70g UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent; No. 3, 5I8,492 Dated June 30, I970 It is certified that error appears in theabove-identified patent and that said Letters Patent are herebycorrected as shown below:

r? II II 1 oI I I me 46, voIrage f rsr occurrence shou I d be volTage s2, I Ines 28 6. 29, "An examp le 01 such a Triggered spark gap assemblies should be dele'fed.

, line 46, inserI known affer "having" line 66, inserT neI'work "beforecomma 7, Iine 70, "impendance" should be--impedance- 8, line 3|, "gap"should be gaps II, line 56, "hunIing" should be shunring I ine 46,"esfablishng" should bees+ablishing su-TEEJ am.) LE3 OCH-1m Aueat:

min: 1: sum .m Attesting Officer Omiuissibner of Patents:

